1. Field of the Invention
This invention relates to a method and apparatus for injection molding.
More particularly the invention concerns the production of injection molded plastic articles by introducing a stream of molten plastic material from an injection molding machine cylinder into an article forming mold space, and supplying pressurized gas into the interior of the molten plastic material to form a cavity therein. When the plastic material has solidified and cooled sufficiently so that the article can itself sustain the form dictated by the mold surface, the gas containing cavity is vented to reduce the pressure within the cavity to ambient pressure. The mold is then opened to allow the molded article to be removed therefrom.
2. Prior Art
It is known to introduce the gas through the injection nozzle which introduces the plastic material, and then to vent the cavity by a step known as "sprue break" in which the nozzle is withdrawn from the mold whereby a gap is created between the nozzle and the mold which allows the gas to pass from the cavity to atmosphere. However, some molding machines are not equipped to permit sprue break. Also, it may be considered undesirable to vent the gas, e.g. nitrogen, into the environment of the molding machine for health reasons. In each case, it would be preferable to vent the cavity at another position in the mold, if desired in a manner in which the gas may be readily channelled to a factory vent.
Forming openings during molding operations is known per se. For example, it is known to generate a high excess pressure to cause a blow-mold to burst in the zone of an orifice to form a desired opening. Such a method would not be an acceptably controlled manner of venting.
It is also known to create a vent opening mechanically in the cavity wall of a molded article by piercing or boring. However, the act of piercing would not itself ensure that the gas would be permitted to escape unless the piercing tool is first inserted and then withdrawn. If a hollow piercing tool is used, venting would be achieved, but it would be difficult to prevent the hollow tool from being blocked by plastic material during the step of introducing the plastic material into the mold space. The same would apply to the provision of a drill for boring a hole in the cavity wall. Thus both previously proposed methods are disadvantageous because they are slow, thereby adding to the molding cycle time, and are impracticable.
It has further been suggested to seal the hole in the cavity wall through which the gas enters the cavity, by injecting a further quantity of plastic material, prior to the step of venting the cavity. Such a step may be disadvantageous because, with the hole blocked, it is no longer possible to alter the gas pressure within the cavity and thereby to maintain control of the gas pressure.
In a still further known arrangement, there is provided in association with the supply of pressurized gas a valve port for opening the cavity to atmosphere and a valve member for closing the port. During the supply of pressurized gas, the valve member is closed but the valve port is maintained open to the cavity. Thereby, after a molding operation has been finished, the cavity can be vented by opening the valve member to connect the still open valve port to atmosphere.
In another known arrangement for injection molding, a pin is positioned in the inner end of a vent passage for movement between an extended position in which the vent passage is closed and a retracted position in which the vent passage is open, and means are provided for moving the pin between its extended and retracted positions. However, in this arrangement, the inner end of the vent passage is not open to a part of the interior of the mold at which the gas containing cavity is to be formed. Also, as in other known arrangements, at the end of the molding operation a vent opening exists in the cavity wall. The vent opening is not created by or on retraction of the pin.